In a cellular network mode, data traffic usually goes through centralized controller such as a base station (BS) even if the communicating devices are close to each other. One benefit of the centralized operation is easy resource control and interference control. One potential drawback is less efficient resource utilization. A double amount of resources may be needed for a user equipment (UE) in a cellular mode. For example, a cellular UE may need to have a first radio resource allocated between the cellular UE and the base station and a second radio resource allocated between the base station and the other cellular UE. Examples of the centralized controller may include evolved Node B (eNB) of long-term evolution (LTE).
In comparison, a UE in a direct, device-to-device (D2D) mode may only need one radio resource between the UE and a pairing UE, when two users are sufficiently close to each other. To help improve system throughput, a radio network may operate in a D2D mode. A UE may operate in the cellular mode if the other party is located far way and may operate in the D2D mode if the pairing UE is sufficiently close by. Mixed cellular and D2D modes are being explored in newer generations of wireless technologies such as 3rd generation partnership project (3GPP) long-term evolution-advanced (LTE-A) and worldwide interoperability for microwave access (WiMax) network. One example of D2D mode network is an ad-hoc network where one D2D UE may set up a direct connection with the pairing UE via a handshake and competition procedure. Examples of cellular networks include widely deployed wireless networks such as universal mobile telecommunications system (UMTS) network, code division multiple access (CDMA) network, WiMax network and 3GPP LTE networks.
In the D2D mode, two D2D UEs may share downlink or uplink resources with cellular mode UEs. These shared links may be referred to as shared uplinks and shared downlinks hereafter. Alternatively the two D2D UEs may use dedicated cellular uplink or dedicated cellular downlink resources for D2D communications. In these resources dedicated for D2D communications no cellular mode communication takes place within a cell, a part of a network or the whole network. These dedicated links may be referred as dedicated uplinks and dedicated downlinks hereafter. Thus a pair of UEs in the D2D mode may be further defined as in one of the following four resource allocation modes: a reusing downlink mode, a reusing uplink mode, a dedicated downlink mode, and a dedicated uplink mode. A special case is that when the two D2D UEs fail to acquire any of the four resource allocation modes, they may turn to cellular mode operation. In this case, the cellular mode may be viewed as a special case of the D2D resource allocation mode. Hence, the term D2D mode refers to one of the D2D resource allocation modes hereafter unless specified otherwise.